Electrical machines needs cooling to dissipate heat, which is generated during its operation by ohmic resistance, by iron hysteresis, etc.
It is possible to cool a small electrical machine by a heat-transfer from the inside of the machine to its surface.
This is not possible for a large machine, which shows a relatively small surface per power rating and heat generation.
When a machine is installed indoor at a dry atmosphere it is possible to operate the machine without a housing, so a cooling is achieved by the circulation of ambient air through the machine.
When the machine is installed under harsh conditions, like it is for generators being used in offshore wind-turbines, the electrical machine need to be totally enclosed, so ambient air is not allowed to circulate through the machine. For this application dedicated cooling-systems are required.
One very common cooling-method is the circulation of air or another gaseous medium inside the electrical machine, while the cooling-medium is kept cool by a heat-exchanger. This cooling method disadvantageously requires large gas-to-air or gas-to-water heat-exchangers. Furthermore considerable additional power is required to circulate the cooling-medium inside the machine.
Another cooling-method of a generator, which shows a stator and a rotor, is the circulation of a liquid on a first side of the stator. This first side to be cooled is opposite to an air gap, which is between the stator and the rotor. The stator shows a number of stacked laminate-plates, which carries metal-windings of stator-coils, so the heat is transferred from the metal-windings through the laminate-plates to the cooling-medium by conduction.
This cooling method suffers from a considerable temperature-gradient, which exists between the windings of the stator and the cooling-medium—due to a moderate heat-conductivity of the laminate-plates. Because of this it is difficulty to maintain a predetermined winding-temperature, which is below a required maximum-value.
Another cooling-method is to bring in liquid or gas for cooling-purposes into slots of the laminate-plates, while these slots are used to carry the metal-windings. To bring in the cooling-medium hollow ceramic-cooling-pipes are used, which are expensive and difficulty to handle.
FIG. 3 to FIG. 5 shows state-of-the-art arrangements for cooling of a generator G, which may be used inside a nacelle of a wind-turbine.
Referring to FIG. 3 the generator G is cooled by normal air A. The cooling-air A is forced to flow through the generator G by help of a fan F and is used inside the generator G for cooling-purposes. The heated air A is leaving the generator G later.
Referring to FIG. 4 the generator G is cooled by air A1, which is forced to flow through the generator G by help of a fan F1.
It is not possible for offshore-locations to use ambient air for the cooling as described in FIG. 3 as ambient air comprises salt-particles.
Therefore a closed system is used to circulate the cooling-air A1 to and from the generator G, while a heat-exchanger HX is used to hand-over the heat from the cooling air A1 to another cooling-system, which also uses air A2 for the transport of heat.
The air A2 of this second cooling-system is forced to flow to and from the heat-exchanger HX by help of a fan F2.
The cooling air A1 is separated from an ambient air A2 by help of the heat exchanger HX, which is a air-to-air heat-exchanger. This results as described in two needed fans F1 and F2.
Referring to FIG. 5 the generator G is cooled by a normal liquid LQ. The cooling liquid LQ is forced to flow through the generator G by help of a pump P.
The cooling liquid LQ is cooled by ambient air A3 by help of a heat-exchanger HX, which transfers the heat to circulated ambient air A3.
The air A3 is forced to flow to and from the heat-exchanger HX by help of a fan F3.
The benefit of the liquid cooling system as shown in FIG. 5 is a higher capacity in intensive cooling of the generator G and furthermore, the generator G is separated from a salty ambient air, comparable to FIG. 4.
For new electrical machines (i.e. as motors and generators) an external magnetising system is replaced by permanent magnets. For these types it is essential to keep the temperatures of the magnets low to achieve a high capacity and efficiency.
There are two basic physical constraints, which must be taking into account to achieve a high cooling capacity—the used cooling-medium as liquid or gas has to show a high speed while passing hot-spots for an efficient cooling. Additionally or alternatively the used cooling-medium has to show a low temperature for an efficient cooling.
To circulate the cooling-medium with high speed induces a high power-consumption needed by pumps of fans, so an overall efficiency of the electrical machine is reduced. Furthermore a high speed circulating cooling-medium may induce unacceptable noise.
There is one problem remaining: if the electrical machine is located at hot sites as deserts it is not possible to cool down the medium to a temperature, which is below the ambient temperature. Additionally if the cooling-medium needs to be cooled down close to the ambient temperature then the size of required heat-exchangers has to grow dramatically.
This relationship is not linear: the size is to a certain extend proportional to the “inverse of the temperature difference”—in this case:A=K×1/dT,whereA is a property for the dimension of the heat exchanger—for example the surface area,K is a constant-value, anddT is the difference between the temperature of the cooled-down medium and the ambient-temperature.